Those of you who followed the news last week may have seen disturbing headlines warning that we may be headed for ocean levels a full 20 feet higher than they are at present—this article from CBS News was typical. The odd thing about this round of press is that we already more or less knew this; there was no actual news.
The articles were prompted instead by a scientific review of what we know about the relationship between rising temperatures and rising oceans that has occurred in the past. Rather than performing a new analysis, the review simply looked at existing studies in the literature and tried to pull them all together into a single, coherent picture. And that picture suggests that ocean levels have been much higher following only a slight rise in temperature.
The problem is that all of the periods that might be informative about the present warming occurred at least tens of thousands of years ago. We can't directly measure things like temperature, ocean levels, or ice sheet volumes. Instead, we have to infer these things from proxies: measurements we can make from samples preserved from this time.
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